WASHINGTON, Sept 1 (Reuters) - U.S. job growth slowed more than expected in August after two straight months of hefty increases, but the pace of gains should be more than enough for the Federal Reserve to announce a plan to start trimming a massive bond portfolio accumulated as it sought to bolster the economy.

Persistently sluggish wage growth could, however, make the U.S. central bank cautious about raising interest rates again this year. The Labor Department said on Friday nonfarm payrolls increased by 156,000 last month. The economy created 399,000 jobs in June and July.

“We see nothing here that prevents the Fed from initiating its balance-sheet reduction plan at the September meeting,” said John Ryding, chief economist at RDQ Economics in New York.

Average hourly earnings rose three cents or 0.1 percent after advancing 0.3 percent in July, keeping the year-on-year gain in wages at 2.5 percent for a fifth consecutive month. The average workweek slipped to 34.4 hours from 34.5 hours in July.

U.S. stocks were trading higher, with the Dow hitting the 22,000 mark for the first time in more than two weeks. The dollar was little changed against a basket of currencies, while prices for U.S. Treasuries fell.

August’s moderation in employment growth, which pushed payroll gains below the 176,000 monthly average for this year likely reflects a seasonal quirk. Over the past several years, the initial August job count has tended to exhibit a weak bias, with revisions subsequently showing strength.

“The August payroll count does tend to be biased downward, typically reflecting seasonal difficulty in measuring the timing of back-to-school, as well as low initial response rates during the summer,” said Robert Rosener, an economist at Morgan Stanley in New York.

The department said Hurricane Harvey, which devastated parts of Texas, had no “discernable” effect on payrolls as the disaster struck after the survey period for the August employment report. Economists say the storm could hurt September payrolls if the disruption from the flooding is prolonged.

The smaller household survey showed a decline in employment last month. As a result, the unemployment rate ticked up one-tenth of a percentage point to 4.4 percent.

Economists had forecast payrolls increasing by 180,000 jobs last month. Still, August’s gains were far more than the 75,000 to 100,000 jobs per month needed to keep up with growth in the working-age population.

The labor market has continued to strengthen even as hopes for a promised tax cut this year have faded.

Republican President Donald Trump on Wednesday reiterated his longstanding call for slashing the U.S. corporate tax rate to 15 percent from 35 percent at a time when lawmakers believe they would be lucky to bring it down to 25 percent.

The Republican-led U.S. Congress faces a tough challenge in passing tax reform legislation, having already failed to deliver on healthcare reform sought by Trump.

FACTORIES HUMMING

Underscoring labor market strength, manufacturing payrolls surged by 36,000 jobs in August, the most in four years, with the motor vehicle sector adding 13,700 positions.

Employment in the sector could be buoyed by an anticipated spike in demand for automobiles as residents in the Houston area replace flood-damaged vehicles.

A second report on Friday from the Institute for Supply Management showed its factory activity index soared to 58.8 in August, the highest reading since April 2011, from 56.3 in July. A measure of factory employment hit a more than six-year high.

The employment report showed construction jobs jumped 28,000 last month. That was the largest gain since February and came despite a lull in homebuilding activity and home sales. Another report on Friday showed construction spending falling to a nine-month low in July.

While August’s job gains likely keep the Fed on course to outline a plan to start shrinking its $4.2 trillion portfolio of Treasury bonds and mortgage-backed securities at its Sept. 19-20 policy meeting, tepid wage growth casts doubts on a December interest rate increase.

The anemic wage gains came on the heels of a report on Thursday showing the Fed’s preferred inflation measure, the personal consumption expenditures price index excluding food and energy, increased 1.4 percent in the 12 months to July - the smallest rise in just over 1-1/2 years.

U.S. financial markets are pricing in a roughly 42 percent probability of a rate hike at the Fed’s December meeting according to CME Group’s FedWatch program. The Fed has increased borrowing costs twice this year.

Lack of strong wage growth raises concerns about the sustainability of a recent surge in consumer spending, which spurred the fastest economic growth in more than two years in the second quarter.

The private services sector led the slowdown in job gains last month, with payrolls rising 95,000. That was the smallest gain since March and followed an increase of 179,000 jobs in July. Economists also blamed the moderation in hiring on a dearth of qualified workers.

“The economy can’t create more jobs as there is no one left to hire at this late stage of the recovery with the expansion starting its ninth year in July,” said Chris Rupkey, chief economist at MUFG in New York.

Retail employment increased by 800 last month as a surge in hiring at building material and garden supply stores was offset by continued layoffs at clothing retailers.

Payrolls at nonstore retailers rose only 700 despite online retailer Amazon.com holding a series of job fairs to recruit about 50,000 workers last month. Government hiring declined for a second straight month in August.